More and more reliance is being placed on data communication networks to carry increasing amounts of data. In a data communications network, data is transmitted from end to end in groups of bits which are called packets, frames, cells, messages, etc. depending on the type of data communication network. For example, Ethernet networks transport frames, X.25 and TCP/IP networks transport packets and ATM networks transport cells. Regardless of what the data unit is called, each data unit is defined as part of the complete message that the higher level software application desires to send from a source to a destination. Alternatively, the application may wish to send the data unit to multiple destinations.
Asynchronous Transfer Mode
ATM originated as a telecommunication concept defined by the Comite Consulatif International Telegraphique et Telephonique (CCITT), now known as the ITU, and the American National Standards Institute (ANSI) for carrying user traffic on any User to Network interface (UNI) and to facilitate multimedia networking between high speed devices at multi-megabit data rates. ATM is a method for transferring network traffic, including voice, video and data, at high speed. Using this connection oriented switched networking technology centered around a switch, a great number of virtual connections can be supported by multiple applications through the same physical connection. The switching technology enables bandwidth to be dedicated for each application, overcoming the problems that exist in a shared media networking technology, like Ethernet, Token Ring and Fiber Distributed Data Interface (FDDI). ATM allows different types of physical layer technology to share the same higher layer--the ATM layer.
More information on ATM networks can be found in the book "ATM: The New Paradigm for Internet, Intranet and Residential Broadband Services and Applications," Timothy Kwok, Prentice Hall, 1998.
ATM uses very short, fixed length packets called cells. The first five bytes, called the header, of each cell contain the information necessary to deliver the cell to its destination. The cell header also provides the network with the ability to implement congestion control and traffic management mechanisms. Fixed length cells offer smaller and more predictable switching delays as cell switching is less complex than variable length packet switching and can be accomplished in hardware for many cells in parallel. The cell format also allows for multi-protocol transmissions. Since ATM is protocol transparent, the various protocols can be transported at the same time. With ATM, phone, fax, video, data and other information can be transported simultaneously.
ATM is a connection oriented transport service. To access the ATM network, a station requests a virtual circuit between itself and other end stations, using the signaling protocol in the ATM switch. ATM provides the User Network Interface (UNI) which is typically used to interconnect an ATM user with an ATM switch that is managed as part of the same network.
Unicast and Multicast Connections
The transmission of data to a single destination is termed a unicast connection while the transmission of data to multiple destinations is termed a multicast connection. For example, in connection with ATM switches, for each call connection within the switch, each cell has associated with it a routing tag that identifies the destination for the cell. For multicast connections, the routing tag includes identification of several destinations for the cell.
The majority of data communication networks today attempt to not lose even a single bit of information. In order to achieve such high levels of performance and low error rates, the network equipment is constructed with relatively large sized queues. The large queues are needed in order to handle the accumulation of cells (or packets, frames, etc.) without overflowing the queues. An overflow of a queue will result in cells being lost, i.e., dropped. Cells may be dropped at the source queue if the destination is full and thus cannot receive additional cells. In this case, the source queue fills up and at some point will overflow, with the consequent cell loss.
The destination queue uses a backpressure signal to indicate to the source queue that the destination queue is full and cannot receive additional cells. This problem is exasperated in multicast connections since each cell in the source multicast is replicated to several destination queues. In prior art data communications devices, the backpressure signal from any output port on the distribution list of the multicast connection will cause cell traffic to cease to the output port that is full and all the other output ports in the multicast group regardless of whether they are congested or not.